Variable air-gap crystal holder



May 18, 1943 G. M. THuRsToN 2,319,357

VARIABLE AIR-GAP CRYSTAL HOLDER /N VEN TOI? @y @.MfHl/Rsroy A 7' TORNE Y May 18, 1943 G. M. THURSTON Filed Sept. 3, 1941 VARIABLE AIR-GAP CRYSTL HOLDER 2 Sheets-Sheet 2 Wvg/v70@ a. M. THU/Ps ro/v l CMM A 7' TORNE Y shape.

Patented May 18, 1943 VARIABLE AIR-GAP CRYSTAL HOLDER George M. Thurston, Hasbrouck Heights, N. J., assignor to Bell Telephone Laboratories, Incor-1 porated, New York, N. Y., a corporation of New York Application September 3, 1941, Serial No. 409,370

Claims.

This invention relates to piezoelectric crystal apparatus and particularly to mounting and frequency adjustment arrangements for piezoelec-v tric crystal elements such as thickness-mode quartz crystal elements useful. in the control of oscillation generators in radio transmitters and receivers, for example.

One of the objects of this invention is to provide piezoelectric crystal apparatus, the circuit frequency of which may be varied within certain limits and adjusted to an exact value.

Another object of this invention is to provide improved means for resiliently clamping a piezoelectric crystal element to hold it rigidly against bodily movement out of a predetermined position with respect to its clamping means while independently providing therefor an. angularly variable air-gap electrode to adjust the circuit frequency thereof to a desired value independently of the clamping means.

The frequency of a piezoelectric quartz crystal element in an oscillator circuit, for example, is a function of the spacing of its surfaces from its adjoining electrodes. In accordance with this invention, by varying the gap or spacing between one of the surfaces ofthe crystal element and its adjoining angularly movable electrode, the frequency of the circuit may be varied within limits and adjusted to a desired frequency.

To mechanically hold the crystal element in position at all times and to reduce undesired extraneous frequencies therein, the crystal element, which may be a thickness-mode crystal element of low or zero temperature-frequency coefli'cien't, for example, may be rigidly and resilintly clamped at its peripheral portion only, and at all four corners thereof when the major surfaces thereof are rectangular or square in The crystal element being adequately clamped isV not adversely affected by mechanical shock or vibration,` and being provided with an angularlyadjustable air-gap electrode that is adjustable independently of the crystal clamping means, may be adjusted to an exact frequency value. VThe crystal unit is relatively inexpensive to manufacture, and may be made of a size that occupies a relatively smallspace.

In a particular embodiment, the crystal element may be provided with corner clamping only and with an independent angularly adjustable air-gap velectrode which is yspringhinged to crystal clampingframe. ralso,v spring pressure'may be applied to crystal electrode assembly in`three mutually perpendicular dlrectionsto prevent bodily movement of the assembly out of a .predetermined position within the enclosing container. In such an arrangement, there is no wear or disturbance of position of the crystal element as a result of adjustments in the air-gap, and the device is particularly useful in mobile applications or wherever small adjustments in frequency are desired.

For a clearer understanding of the nature of this invention and the additional advantages. features and objects thereof, reference is made to the following description taken in connection `with the accompanying drawings, in which like reference characters represent like or similar parts and in which:

Figs. l, 2 and 3 arerespectively front, side and top views, in section, of a piezoelectric crystalholder unit in accordance with this invention, Fig. 1 being a view taken on the line I--I of Fig. 2, Fig. 2 being a view taken on the line 2-2 of Fig. 1, and Fig. 3 being a top view of Fig. 1;

Fig. 4 is an exploded perspective view of parts of the `crystal holder unit illustrated in Figs. 1 to 3; and

Fig. 5 Ais another exploded perspective 'view of parts o fl the 'crystal holder unit illustrated in Figs. 1 to 4.

Referring to the drawings, the crystal holder device, as illustrated in Figs. 1 to 5, may com-` prise a housing composedof insulating material suchas molded Bakelite and consisting of a container I0 of nearly cubical shape with covers II and I2 therefor composed of the same material. The cover I2 may be removably fastened to the container casing III by screws such as the screws I4 vinsertable through corresponding circular openings in the cover I2 and engaging corresponding internally-tapped brass inserts I8 embedded in the phenol productcasing i0. A nameplate II may be removably secured to the cover I2 by screws insertable through openings in the nameplate I1 and engaging corresponding internally-tapped brass inserts embedded in the cover I2. The contents of the container I0 are made accessible upon removal of the screws that -hold the nameplate I'I to the cover I2, and upon removal of the screws Il that hold the cover I2 to the container body i0.

As illustrated in Figs. 1 and 2, three hollow tubular metal terminal pins or prongs 20, 2I and 22,'each having rounded ends or tips 25, may be secured to the outside bottom surface of the container body I0 and utilized for mechanically supporting the device and also for establishing external electrical connections therewith. As ilyto a fixed or stationary frame or plate 52.

lustrated in Figs. 1 and 2, wires or rods 23 and 24 may be soldered to the ends or tips 25 of the terminal pins 20 and 2| and disposed within the hollow pins 20 and2l to individually establish the electrical connections with a piezo-electric crystal element 40 which is `disposed between a stationary electrode 50 and an angularly movable electrode the latter being spring-hinged A wire 24 may be soldered at one end thereof to the fixed plate 52 and at its other end to the pin terminal 20, and a rigid rod 23 maybe soldered at one end'thereof to the pin terminal 2| and at its other end to the conductive connector plate 54 disposed in contact with a conductive V-shaped spring 55 which electrically interconnects the connector plate 54 with the stationary electrode plate 50. As illustrated in Figs. 1 to 4.

the contact plate 54 may be placed against the side and bottom inner walls of the container I0 to electrically interconnect the external terminal pin 2| with the V-shaped spring 56 and the stationary electrode 50. The crystal clamping electrode plate 50 and the V-shaped spring 56 may be of the'form described in United States Patent 2,115,145 to L. F. Koerner, dated April 26, 1938, Figs. 1 to 4. i v

Varnished tubing or other suitable insulated conductor wire may be-utilized to interconnect the pin terminal withthe stationary frame or plate 52, the opposite ends of the wire 24 being soldered or otherwise conductivelyvattached to the terminal pin 20 and to the plate 52. By means of the plug-in type terminals 20, 2| and 22v and corresponding sockets therefor (not shown), the entire unit may be removably inserted in the oscillator circuit of a radio transmitter or receiver, for example, to control the frequency of oscillations at a desired value in accordance with the adjusted frequency characteristic of the crystal element 40.

The crystal element 40 may be a thicknessmode quartz crystal plate of the type described, for xample, in United States Patent 2,218,200, granted October 15, 1940, to F. R. -Lack et al., which includes an AT cut quartz crystal element 40-of the low temperature-frequency coefficient f type, the frequency of which may be varied within about :L-.025 per cent of its nominal frequency by adjustment of a calibrated'dial 80 which may be reset to a value of .001 per cent. The nomp inal frequency of the crystal element 40 may be a value within a range of the order from1600 or less to 4500 or more kilocycles per second. for example.

The spring-pressed electrode plate 50 may have at each of the corners of its major surface that is adjacent the crystal element 40 four integral fiat-surfaced corner projecting lands 53 .which are adapted to make contactwith one of the maior surfaces of the crystal element 40 only at the four corners thereof, the'central depressed portion between the corner projections 53 being a at surface which may be uniformly spaced about .001 inch from the adjacent major surface of the crystal element 40, as described in the hereinbefore mentioned L. F. Korner United States Patent 2,115,145, dated April 26, 1938.

The stationary center plate 52 which may be removably slidable in two opposite grooves 6| cut or molded in the opposite .interior walls of the container i0, mechanically divides the container I0 into two compartments, in one of which on one side of the center plate 52 is placed the crystal element 40, the crystal electrode plate 50, the

V-shaped spring 56, the connector plate 54, and a removable insulating spacer 58, which is disposed in slidably fitting relation between the parallel plates 52 and 54 adjacent the lower edges of the crystal element and its stationary electrode 50.

The container i0 may interchangeably hold a crystal element 40 and an electrode 50 of various corresponding sizes and shapes. For this purpose, a suitable removable insulating spacer 58 having external dimensions suited to the dimensions of the compartment between the parallel plates 52 and 54 and having internal configurations suitable for the edges of the electrode 50 and the crystal element 40 may be provided.

-Where the crystal element 40 and the electrode 50 are of rectangular shape. the insulating spacer 58 may be shaped as illustrated in Figs. l and 4.

The spring 56 may be in the form of a V- shaped spring and may have a central rib 51 to permit the spring 5S to be readily there bent and adjusted to exert a suitable clamping pressure on Athe four corners of the crystal clement 40. The spring 56'is constructed and placed in con-l tact with the electrode plate 50`in a manner to register with and exert pressure only on the four corner clamping projections 53 of the electrode plate 50. The V-shaped spring 56 may be adjusted to exert a pressure of approximately five pounds on the crystal element 40, or of other suitable value to suit the thickness of the crystal element 40. and to rigidly clamp the crystal element 40 so that the V-sliaped spring 56 may at all times press the crystal element 40 against the fixed center plate 52.

A bow spring 43 which may be made of spring steel or of other suitable spring material may be placed in the groove of an insulating guide of moulded Bakelite, for example, and utilized to apply spring pressure in edgewise direction to the electrode assembly that is located at one side of the center plate 52. Also, a coil spring 41 may apply pressure thereto in another direction acting on the insulating spacer 58, the V- shaped spring 56 applying pressure thereto in a third direction. Accordingly,' by means of the springs 43, 41 and 56, spring pressure may be applied to the electrode assembly 50 in three mutually perpendicular directions to prevent bodily movement of the assembly out of a predetermined position within the container I0. A

bow spring48 may apply spring pressure to the center Aplate and bracket assembly 52, 83 to prevent bodily movement thereof out of a predetermined position against a wall of the container i0.

59 attached to the marginal rectangular shaped frame 52 which is located on the other side of they crystal element 40 adjacent the peripheral margin thereof. The four clamping projections 59 may be in the form of studs pressed into place in suitable openings cut near the four corners of the plate frame 52.

Accordingly, the piezoelectric crystal element 40 is resiliently held in position between the four example, roughly oi' the order of clamping projection 58 of electrode plate 50 and the four oppositely disposed clamping projections 58 which are secured to the center plate 52 adjacent the four corners of the crystal element Attached tothe lower margin of the stationary center plate 52 is a flat spring 12 which is also attached by screws 18 to the lower edgeof the angularly movable electrode the latter freely passing through a rectangular opening 16 in the center of the plate 52 and being disposed closely sprung away from the xed plate 52 constantly holds the extension plate 15, that is secured by welding or otherwise to angularly movable electrode 5|, against the hump 18 on the ilat face of an adjustable control knob 80 which is rotatable on the screw-threaded stud 82 attached by screws 84 to a bracket 83 carried by the center plate 52.

The spring 12 may be made of phosphor-bronze or other suitable spring material and may be riveted at two points or'otherwise secured to the bottom part of the fixed center plate 52 in such a manner that the spring 12 is normally biased or sprung away from the fixed plate 52 in a direction away from the crystal element lil. The control knob 8U engaging the stud screw 82 attached to bracket 83 carried by plate 52 may be rotated on the screw 82 and by contact of the hump 18 with the arm 15 of the angularlv movable electrode5| squeeze the electrode 5| angularly towards the crystal element 40.

' The adjustable control knob 80, when rotated inwardly on the stud screw 82, works against the pressure of the spring 12 and causes the angularly movable electrode 5| to be moved towards the crystal element 4|! thereby decreasing and varying the angular air-gap between the crystal element lll and the movable electrode plate 5| and causing a frequency variation in the crystal unit. 'Ihe pitch of the thread of the screw 82 may be made of such a value that Ithe y control knob 80 Vmay make one complete rotation in order to move theelectrode 5| through its entire adjustment distance, which may be, for

1,/8 inch or less at the upper edge thereof.

The pressure exerted by the spring 12 against.l

the hump 18 of the control knob 80 may be such as t'o provide suiiicient friction against a too easy turning of the control knob 80 from any desired setting'.` If desired, additional means such as a electrode 5|l which extends through the opening in the center of the iixed plate 52.

The ilxed plate 52, the bracket 83, and the stud 82 secured thereto may be composed of nickel-silver or other suitable material. The spring 12 may be made of phosphor-bronze, spring tempered. l'I'he control knob 85 may be made of nickel-silver provided with a knurled` periphery 81.

The angularly adjustable velectrode 5| is at all times out of contact with the crystal Delement 4l and at no time clamps the crystal element 40 or otherwise disturbs the position of crystal element 40 or its calibration. To prevent moving the electrode 5| into pressure contact with the crystal element lll, suitable stop and indicating means may be applied to the control knob 80 to conne Y its extreme motion within a limiting 'point beyond which it should not be moved.

The spacing between the outer edges of the movable electrode 5| and the edges of the frame 52 that form the rectangular opening 15 therein may be equal and of a value to permit free movement of the electrode 52 within the opening 18.

'Ihe inner major surface of the movable electrode 5| and that of the peripheral marginal frameplate 52 that face towards the crystal element 40 are non-aligned'with respect to each other, the electrode 5| being disposed nearest to the crystal element I0 illustrated in Figs. 1 and 3.

I'he angularly adjustable rectangular-shaped electrode 5| being spring-hinged to one side of the marginal frame 52 is adjustably movable in the manner of a trap-door within the rectangular-shaped central opening 16 in `the marginal frame 52, thereby to adjustably vary the wedgeshaped air-gap spacing between the crystal element 40 and the movable electrode 5| for adjustclamping the' crystal element 40 and to support y the movable electrode 5|. The spring 12 may be riveted or otherwise .securely attached to the bottom region of the iixed plate 52, the opposite end of the spring 12 being secured to the movable support the spring 12 that ing the frequency of the crystal unit to a desired The peripheral frame plate 52 and the four clamping projections 58 attached thereto may be constructed of metal or of any other material having suitable mechanical properties to function to support and rigidly clamp the crystal ele-v ment 40 at its four corners only, and also to supports and supplies potential to the central electrode 5| attached thereto.

The rigidly clamped corners of the crystal element l0 are quiescent at all times, and the eiective electric iield for operatively driving the desired piezoelectric motion of the crystal element l any useful piezoelectric motion in the thicknessmode crystal element 40 at the periphery thereoi', the useful motion therein to obtain, the desired frequency being at the central portion thereof and resulting from the field applied thereto by the centrally located, electrode 5|.

If it be desired to take the electrode potential off the metallic marginal frame 52, insulators (not shown) may be provided and interposed between the spriig! and the frame 52-in order to electrically insulate the metal frame 52; or alternatively the frame 52 itself may be made of insulating material of suitable mechanical properties.

field-producing movablev With the control knob 80 rotated so that it holds the stop l just ush against vthe plate frame 52, the surfaces of the four clamping studs 59 that contact-the crystal element 40 may be lapped flush with respect to each other and also with respect to the surface of the movable electrode 5i that faces the crystal element 40. After such surfaces have been so lapped, the control knob 80 may be backed olf about of a turn and this position suitably marked to indicate the limit of travel of the dial 80.

Access to the interior of the unit may be had by\unscrewing the two top screws M to remove the name plate Hand unscrewing the screws to remove the cover I2 and the gasket 92. By loosening the screw 89 holding the dial locking device 85, the lock 85 may be swung around so as not to interfere with turning of the dial 80. The dial 80 then may be turned to the desired per cent deviation or frequency setting, and the lock 85 reset. The dial 80 should not be turned beyondits freely movable or adjustable limits in order to avoid any straining of the frame of the unit and displacement of the frequency calibration thereof.

The top plate l I of phenol bre may be secured to the container body Ill by screws and may be provided with a rectangular-shaped opening 90 therein to permit access to the rotatable control knob 8D when the cover l2 has been removed from the container body I0. Gaskets 92 of rubber, or artificial rubber, such as Neoprene or other screws l.

'Ihe clamping projections 53 of the electrode 50 and the clamping projections 59 of the center plate 52 rigidly clamp the crystal element d adjacent its four corners only, independently of adjustments in the angularly movable electrode 5|. to hold the crystal element against the fixed plate 52. The control knob 85 working against the pressure of the spring '12, independently adjusts the air-gap spacing between the crystal element l0 and the electrode plate 5|. f

The crystal element d0 is, by means vof the V-shaped spring 56, 4held in contact with the clamping projections 53 and 59 of the plates 5D and 52 at all times, the movable electrode 5| moving with respect to the xed plate 52 to form an angular or wedge-shaped air-gap, the hinge or pivot points of which are well within the projected major surface area of the crystal element Q0 thus giving a relatively large change in angle for a relatively small movement of the airgap adjustment, and also because of the wedge shape of the air-gap, reducing undesired resonances that might result from a parallel airgap between the crystal surface and the electrode 5I spaced therefrom. The independentrigid and resilient clamping of the crystal element d at its four corners is provided at all times' to rnechanically hold the crystal element in posi- 'tion and to reduce extraneous frequencies in the The V-shaped spring 55 tends at all timesA in the air-gap may be used to change the frequency to either side of a frequency assignment for the station in order to avoid beat-note Yinterference for two stations operating on the same frequency assignment or to operate two stations on the same assignment without mutual interference. The frequency variation of the` crystal element Ml'may be made of the order of about 1*:.03 per cent when the wedge-shaped air-gap is Varied withinthe range resulting from about one turn of the control knob 80. The dial 86 may be calibrated in terms of frequency or wavelength and any setting from minimum to maximum air-gap adjustment may be duplicated. Since the frequency of the crystal unit may be adjusted by movement of the hinged door elec-` trode 5I- to about i025 `of the nominal freit is to be understood that it is capable of application in other organizations and is, therefore,

not to be limited to the particular embodimentsl disclosed, but only by the scope of the appended claims and the state of th prior art.

What is claimed is:

1. -A frequency-controlling piezoelectric crystal device comprising a pair of stationary metallic plates between which a thickness-mode rectangular-faced piezoelectric crystal element is disposed,

means including `a V-shaped spring disposed in contact with one of said plates for resiliently eX- erting pressure on one of said plates and rigidly clamping all four of the corners of the peripheral portion only of the crystal element between said plates, another spring securedto the other of said plates and sprung away therefrom in a direction away from the crystal element, a rectangular-shaped movable electrode carried by said another spring and angularly movable within a rectangular opening in said other of said plates, means including said another spring for pivotally establishing said angular relation between the crystal element and said movable electrode, and

Vmeans including a screw-threaded rotatable member working against the pressure of said another spring for adjustablyincreasing and decreasing the angle between the crystal element and said movable electrode to control the value of vsaid frequency. f

2. A frequency-controlling piezoelectric crystal device comprising a pair of plates between which a thickness-mode rectangular-faced piezoelectric crystal element is disposed, one of said plates having an opening adjacent the central portion only of the major surfaces of the crystal element, means including a spring cooperating with one of said plates for resiliently exerting pressure on and rigidly clamping all four corners of the peripheral portion only of the crystal element between said plates, another spring secured to the other of said plates and sprung away therefrom in a direction away from the crystal element, means including a movable electrode carried by said another spring and movable within said opening for pivotally establishing an anf gular relation between the crystal element and said movable electrode, and means including a rotatable member working against the pressure of said last-mentioned spring for adiustably increasing and decreasing the angleV between the crystal element and said movable electrode to adjust the value of lsaid frequency.

3. A frequency-controlling piezoelectric crystal device comprising a pair of plates between which a thickness-mode piezoelectricl crystal element is disposed and clamped, means including a spring cooperating with one of said plates and with clamping projectionson both ofA said plates for crystal element and said movable electrode,`and

means including a rotatable member working against the pressure of said last-mentioned spring yfor adjustably increasing and decreasing the angle between the crystal element and said movable electrode to adjust the value of said frequency.

4. A frequency-controlling piezoelectric crystal device comprising electrode plates for a piezoelectric crystal element disposed therebetween, a marginal frame surrounding the periphery of one of said electrode plates, a pair of springs disposed on opposite sides of the crystal element,

means including said frame and one of said electrode plates cooperating with one of said springs for resiliently clamping the peripheral portion only of the crystal element, means including the other of said springs carried by said frame for pivotally establishing an angular relation between the crystal element and the other of said electrode plates. and means including a rotatable'member working against the pressure of said other of said springs for adjustably increasing and decreasing the angle between the crystal element and said other of said electrode plates to adjust the value of said' frequency.

5. A frequency-controlling piezoelectric crystal device comprising a stationary'electrode and a movable electrode for a piezoelectric crystal element disposed therebetween, means including a spring disposed on one side of the crystal elef ment and secured to said movable .electrode for pivotally mounting said movable electrode angularly with respect to the crystal element and said stationary electrode, means including' any other spring and said stationary electrode for exerting clamping pressure. resiliently on .the peripheral portion only oi' the major surfaces of the crystal element, said first-mentioned spring tending to increase the angle between the major surface of the crystal element and said movable electrode, and means including a rotatable member cooperating with said first-mentioned spring for adjustably increasing and decreasing said angle between the crystal element and said movable electrode to adjust the value of said frequency, said last-mentioned means being adjustable independently of 'said clamping pressure exl erted on said crystal element.

6. A piezoelectric crystal device comprising a stationary electrode plate and a movable electrode lplate for a crystal elementdisposed therebetween, a frame surrounding the periphery of said movable electrode, means including said stationary electrode plate pressed by a spring forlclampingframe, said clamping of the crystal element being adjacent the peripheral edges only thereof, another spring having its end portions secured to said lframe and to said movable electrode ad-vv jacent a peripheral edge thereof, said last-men? tionedspring being sprung away from said stationary frame in a direction away from the crystal, means including a rotatable member cooperating with said last-mentioned spring for adjust- `plate disposed adjacent the center of the crystal element, `means including a spring for pivotally moving said movable electrode plate angularly in a direction to increase the angle between the crystal element and said movable electrode plate, said movable electrode plate being pivoted at a region within the projection of the major surface area of the crystal element, said springbeing secured at one of its ends to said stationary plate and having its opposite end secured to said movable electrode plate at that -part thereof that is adjacent said pivoted region thereof, said spring being sprung away from said stationary plate in a direction away from the crystal element, and a rotatable adjustable member carried by said stationary plate and cooperating with said spring secured to said stationary plate for adjustably increasing and decreasing the angle between said major surface of the crystal element and said movableelectrode plate to adjust the value of said frequency.

8. A frequency-controlling piezoelectric crystal device comprising a stationary electrode plate disposed on one side of the crystal element, a movable electrode plate disposed on the opposite side of the crystal element, a frame having an opening therein containing said movable electrode lplate, means including a, spring for clamping the periphery of the crystal element between said frame and said stationary electrode plate, said Amovable electrode plate being pivoted at a region to said frame, said movable electrode plate exthe crystal element with respect to said stationary tending through said opening in said frame and being out of contact with the crystal element. said another spring being sprung away from said frame in a direction away from the crystal element, and a screw-threaded rotatable adjustable -member carriedV by said frame and working against the pressure of said last-mentioned spring for adjustably increasing and decreasing the angle between said major surface of the crystal element and said movable electrode plate to adjust the value of said frequency.

9. A frequency-controlling piezoelectric crystal device comprising a stationary rectangularshaped electrode plate of the same size as and disposed on one side of the crystal element, a movable electrode -plate of smaller size than and disposed on the opposite side of the crystal element, said crystal element havinga rectangularshaped major surface, a stationary frame surrounding the peripheral edges of said movable electrode plate, means including a V-shaped spring engaging said stationary electrode plate for clamping the periphery of the crystal element between said frame and said stationary electrode plate, said movable electrode vplate being pivoted at a region within and not beyond the projection of the major surface area of the crystal element by a hat spring having one of its edges secured to said frame and having its opposite edge secured to said movable electrode `plate and disposed out of contact with the crystal element, said at spring being sprung away from said frame in a direction away from the crystal element, said V-shaped spring and said atshaped spring being disposed on opposite sides of the crystal element, said V-shaped spring resiliently exerting clamping pressure on-the peri-phery only of the crystal element and on the four corners thereof, and a screw-threaded rotatable adjustable member carried by said stationary frame and acting against thel pressure of said dat-shaped spring for adjustably increasing and decreasing the angle between said major` surface of the crystal element and that of said movable electrode plate to adjust the value of said frequency.

10. A piezoelectric crystal holder for a thickness-mode crystal element comprising a con-- tainer, a stationary electrode and a movable electrode between which a piezoelectric crystal element is disposed in said container, a frame having an opening in which said movable electrode is disposed, means including a V-shaped spring engaging said stationary electrode for clamping the periphery of said crystal Aelement between said frame and said stationary electrode, said movable electrode being pivoted with respect to said frame at a region within the projection of the major surface area of said crystal element by means of a nat-shaped spring secured to said movable electrode and to said fra-me 'at said pivoted region thereof, said nat-shaped spring -being sprung away from said frame in a direction away from said crystal element, said V-shaped spring and said dat-shaped spring being disposed on opposite sides of said crystal element.

a screw-threaded rotatable adjustable member carried within said container and opposing the action of said flat-shaped spring for adjustably increasing and decreasing the angle between the maior surface of said crystal element and that of said movable electrode, and means including said springs for individually establishing external electrical connections with said electrodes.

11. A piezoelectric crystal device comprising a spring-controlled movable electrode. plate, a frame and a stationary electrode plate between and by which a piezoelectric crystal element having a rectangular major surface is resiliently clamped adjacent the Yfour corners thereof, a spring secured to said movable electrode plate and pivoted to said frame, said spring being sprung away from said frame in a direction away from the crystal element, and a rotatable member carried by said frame and working againstA the pressure of said spring for adjustably increasing A Cil and decreasing the angle between the major surface of the crystal element and that of said movable electrode plate to vary the frequency of the crystal element. positioned between said electrode plates.

12. An adjustable air-gap piezoelectric crystal holder including a pair of electrodes, a frame having an opening therein through which one of said electrodes is movable, means including a spring for clamping the periphery of a crystal element between said frame and one of said pair of electrodes, and means including a screw-controlled spring hinged to said: frame and said movable electrode for angularly adjusting to a desired value the spacing of said crystal element i with respect to said movable electrode.

13. Piezoelectric crystal apparatus comprising an electrode adjacent one major face of the crystal element, meansincluding a frame disposed adjacent the margin of the opposite major face of the' crystal element for resiliently clamping the corners of the crystal element between said frame and said electrode, and a movable electrode angularly hinged to said frame and disposed within an opening therein and forming with said opposite face of the crystal element an air-gap which is adjustable independently of said clamping means.

14. Piezoelectric crystal apparatus comprising an electrode disposed adjacent one major face of the crystal element, a marginal frame disposed adjacent the periphery only of the opposite major `face of the crystal element, means for resiliently clamping the crystal element between said electrode and said frame,said frame having an opening therein, said opening being adjacent the center only of said major faces of the crystal element, a movable electrode spring-hinged to said frame and angularly movable within said opening, and means for adjustably varying the angular spacial relation of said movable electrode with respect to the crystal element.

of said plate for moving said electrode away from said crystal element, adjustable screw means for moving said movable electrode toward said crystal element against the pressure of said spring, and means including a plurality of springs for exerting pressure on said stationary electrode in three mutually perpendicular directions to resiliently x the position of said stationary electrode and crystal element with respect to said plate.

GEORGE M. THURSTON. 

